Module loading upgrade

This all started because I wanted the UnixCompleters module and
initialization to move to the base.linux package.

* MyConfig.class modified to load from all modules
* Test-MyModules suppress empty entries
* removed unix entries from base/src/modules.json
+ created base.linux/src/modules.json

base.linux/profile.d/UnixCompleters.ps1

@@ -0,0 +1,4 @@
+Get-Command Import-UnixCompleters -ErrorAction SilentlyContinue | ForEach-Object {
+  Import-UnixCompleters
+  Set-UnixCompleter -ShellType Bash
+}

base.linux/src/modules.json

@@ -0,0 +1,5 @@
+{
+  "Unix": [
+    "Microsoft.PowerShell.UnixCompleters"
+  ]
+}